If I decide to sell a rental property, What do I have to let tenant know

You should let the tenant know the property is being sold and let them know the day of closing where their next rent payment goes. Depending on the duration of the lease, the buyer is required to honor the lease through its end date. However, if the lease is month to month, the buyer can, after closing, bring an action to evict or sign their own lease with the existing tenant. If you like the tenant, given the market, you might negotiate, as part of the sale, that the tenant be given a lease as a condiction of closing.  Best of luck to you.